It is desirable to be able to secure a door, from the interior side of the door, even when the door is locked by means of a deadbolt. This requirement results from the fact that, even when doors are locked in this manner, there is a risk that an unauthorized intruder may obtain possession of a key, which would enable him to release the lock from the exterior of the door.
It is, therefore, conventional to provide, on the inside of a door, one or more bolts, in addition to a deadbolt lock, so that the door can be secured against such an intruder. However, the addition of such bolts is difficult, especially on metal doors and frames, and inconvenient in that the bolts are sometimes unsightly and/or inconvenient to operate and/or costly.
It is, therefore, desirable to be able to lock the deadbolt from the interior side of the door and in such a manner that the deadbolt cannot be released by a key from the exterior of the door.
In Canadian Patent 496,746 issued Oct. 13, 1953, there is disclosed an auxiliary locking means for use in conjunction with a lock knob spindle for the actuation of a spring bolt in a standard door lock. The locking means comprises a flat latch plate member which is located at the interior side of the door surface and which is provided with central hole through which passes the lock knob spindle, which is square. The central hole in the latch plate has two communicating portions, one of which is circular in outline and the other of which has a substantially rectangular outline. The spindle can rotate in the circular portion but, when located in the other portion, is held against rotation by the latch plate member. The latch plate member can be slid to and for to locate the spindle in either of these portions and, thus, to lock and unlock the square spindle.
Australian Patent 147,426 issued Jan. 3, 1949, also discloses a latch mechanism which enables a doorknob spindle to be releasibly locked by the sliding displacement receiving the spindle.
A further example of such a locking mechanism is disclosed in U.S. Pat. No. 2,929,651 issued Mar. 22, 1960 to P. Friedman et.al., which is intended for storm doors.